> >
> > You've got to be kidding - 333ma is nearly the rated PLATE current!
>
> - I am not kidding, Joe, and neither is either Ohm's Law or
> the Eimac spec sheet. The rated plate / anode current for a
> pair of 8875s for SSB is 1000mA. Is 333mA "nearly the rated
> PLATE current" ?
OK ... it's half the rated plate current (Svetlana spec sheet
for their 3CX400/8874 is 350 mA plate current). Still, there
is absolutely no way the 8873/8874/8875 will survive 150 mA of
grid current per tube or an 8877 survive 300 mA of grid current.
> Eimac autopsies of the two 8875s I sent them from a MLA-2500
> showed they died from internal leakage caused by gold sputtering
> off from the grid.
You keep saying that ... based on one letter from a low level
employee who was not qualified or authorized to be making any
assumptions on the cause of the gold stripping.
As you know from your own tube autopsies, the grids in the
small external anode tubes are quite fragile ... much lighter
than the grids in glass bottles of the same power rating ...
and can withstand much less grid current.
Unless the grids are made of materials designed to withstand
very high temperatures, e.g. pirolytic graphite, they are
very easily damaged by excessive dissipation. This is not
the time or place to get into the weak bonds between the gold
plating and the base metal of the grid structure, current
concentration in the plating because of skin effects, and
other phenomena that contribute to plating loss in tubes
with excess grid current. They have all be raised before
but they don't fit your unique theory of parasites so you
ignore them.
You can stick with tetrodes with handles in class AB1
operation (no grid current) and avoid the issue.
73,
... Joe, W4TV
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|